1. Field of the Invention
The present invention relates to a high-frequency switch for changing over between signal paths in high-frequency circuits such as, for example, portable telephones, and in particular relates to high-frequency switches having four ports and employing diodes.
2. Description of Related Art
With portable telephones, two antennas or one antenna and one external terminal are sometimes shared between a transmitter and a receiver. The kind of switch circuit shown, for example, in FIG. 5, has conventionally been used in such structures.
A switch circuit 51 has a structure including two three-port switches 52 and 53 connected to each other. The switch 52 has a first port P21, a second port P22 and a third port P23 and the switch 53 similarly has first to third ports P31, P32 and P33. An antenna ANT is connected at the second port P22 of the switch 52 and the third port P23 is used as the external connection terminal EXT, with a second antenna being connected at this third port P23. That is, this switch circuit 53 may be used with vehicle-mounted portable telephones for connecting an antenna installed in the vehicle as a second antenna. Further, in order to measure electric characteristics of a receiver implemented in a portable telephone having such a switching circuit, the third port P23 may be used as an external terminal and a predetermined test signal may be applied to the port.
The structure of the switch 52 is such that the first port P21 may be switched over between the second port P22 and the third port P23. The first port P21 is connected to the first port P31 of the switch 53.
The first port P31 of the switch 53 is constructed so as to be capable of being changed over between the second port P32 and the third port P33. The second port P32 is connected to the transmitter Tx and the third port P33 is connected to the receiver Rx.
It is then possible to connect either the antenna ANT or the external terminal EXT to either the transmitter Tx or the receiver Rx.
A high-frequency switch using diodes shown in FIG. 6 is well known as a component for constructing the three-port-type switches 52 and 53. The high-frequency switch 61 has first to third ports P61 to P63 corresponding to the first to third ports P21 to P23 and P31 to P33. The port P61 is connected to the cathode of a diode 65a via a capacitor 64 and the anode of the diode 65a is connected to a second port P62 via a capacitor 66a. Further, one end of a distributed parameter line 67a is electrically connected at a connection point A between the anode of the diode 65a and the capacitor 66a. This distributed parameter line 67a is of a strip line structure of length .lambda./4 or less where .lambda. is taken to be the wavelength of a high-frequency signal flowing through the switch 61. The other end of the distributed parameter line 67a is connected to a ground potential via a capacitor 68a. Further, one end of a resistor 69a is connected to a connection point between the distributed parameter line 67a and the capacitor 68a and the other end of the resistor 69a is connected to a control voltage terminal VC1.
Further, the first port P61 is connected to a distributed parameter line 71 constructed in the same way as the distributed parameter line 67a via the capacitor 64, with the other end of the distributed parameter line 71 being connected to a ground.
Moreover, the cathode of a diode 65b is connected to the first port P61 via the capacitor 64 and the anode of the diode 65b is connected to the third port P63 via a capacitor 66b. A series circuit comprising a distributed parameter line 67b and a capacitor 68b is also connected across the anode of the diode 65b and ground in the same way as on the side of the diode 65a. Also, one end of a resistor 69b is connected to a connection point between the distributed parameter line 67b and the capacitor 68b and the other end is connected to a control voltage terminal VC2.
With the high-frequency switch 61, the port P61 can be connected to either the second port P62 or the third port P63 by applying different control voltages to the first control voltage terminal VC1 and the second control voltage terminal VC2. For example, if a positive control voltage is applied at the control voltage terminal VC1, while on the other hand, a negative control voltage is applied at the control voltage terminal VC2, a forward bias is applied to the diode 65a and a reverse bias voltage is applied to the diode 65b. A control current supplied from the control voltage terminal VC1 is blocked by the capacitors 66a, 68a, 64, 66b and 68b which restrict the flow of direct current. A current therefore flows in the circuit portion including the distributed parameter line 67a, the diode 65a and the distributed parameter line 71, and the diode 65a is turned on. On the other hand, on the side of the diode 65b, the diode 65b is turned off because a reverse bias voltage is applied to the diode 65b.
Further, with regard to a high-frequency signal provided from the second port P62, the capacitance 68a can be large because the distributed parameter line 67a is provided as described above. The impedance of the series circuit including the distributed parameter line 67a therefore becomes infinite at the connection point A, and therefore a high-frequency signal applied to the second port P62 proceeds to the first port P61.
On the other hand, when a negative control voltage is applied to the first control voltage terminal VC1 and a positive control voltage is applied to the second control voltage terminal VC2, a reverse bias voltage is applied to the diode 65a and a forward bias voltage is applied to the diode 65b. The diode 65a therefore goes off and the diode 65b goes on. There is therefore no signal flow between the second port P62 and the first port P61 and no signal flow between the first port P61 and the third port P63. There is also no high-frequency signal flow through the distributed parameter line 67b because the impedance of the series circuit comprised of the distributed parameter line 67b and the capacitor 68b is infinite at the connection point B.
The distributed parameter lines 67a and 67b form current paths for the control current to flow through the diodes 65a and 65b. Therefore, with respect to high frequencies, the impedance of the distributed parameter lines 67a and 67b at the connection points A and B can be made high and the insertion loss and reflection loss can be reduced.
In this way, the first port P61 can be switched over between the second port P62 and the third port P63 in the high-frequency switch 61 by applying positive and negative control voltages to the control voltage terminals VC1 and VC2.
The switch circuit 51 shown in FIG. 5 is constructed in such a manner that one high-frequency switch 61 is used as each of the switches 52 and 53, that is, two three-port high-frequency switches are used with their first ports being connected in common.
Because the switch circuit 51 is constructed from two connected switches 52 and 53, high-frequency signals pass through the two switches. For example, a transmission output from the transmitter Tx passes through the two switches 53 and 52 before being provided to the antenna ANT. Similarly, a high-frequency signal inputted from the antenna ANT passes through both the switches 52 and 53 before being provided to the receiver Rx. The insertion loss therefore becomes large. Because of the large insertion loss, the transmission output at the time of transmission has to be increased, and the gain is reduced during receiving. It is therefore strongly recommended for the insertion loss to be reduced.
Moreover, control voltages have to be applied to the first and second control terminals of each of the switches 52 and 53 in the switch circuit 51 because the switches 52 and 53 are constructed using high-frequency switches 61, so that two power supplies for supplying control voltages are necessary for each of the switches 52 and 53. This makes it necessary to form a complicated power supply circuit pattern on a substrate.
It is therefore an object of the present invention to provide a high-frequency switch having four ports that is capable of connecting either of the two ports on one side or the other to either of the remaining two ports, that causes a small insertion loss, and that has a reduced number of constituent parts.